Electrically conductive organic polymers have been of scientific and technological interest since the late 1970's. These relatively new materials exhibit the electronic and magnetic properties characteristic of metals while retaining the physical and mechanical properties associated with conventional organic polymers. Examples of electrically conducting polymers are polyparaphenylene vinylenes, polyparaphenylenes, polyanilines, polythiophenes, polyazines, polyfuranes, polythianaphthenes polypyrroles, polyselenophenes, poly-p-phenylene sulfides, polyacetylenes formed from soluble precursors, combinations thereof and blends thereof with other polymers and copolymers of the monomers thereof.
Conducting polymers are conjugated systems which are made electrically conducting by doping. The doping reaction can involve an oxidation, a reduction, a protonation, etc. The non-doped or non-conducting form of the polymer is referred to herein as the precursor to the electrically conducting polymer. The doped or conducting form of the polymer is referred to herein as the conducting polymer.
Conducting polymers have potential for a large number of applications in such areas as electrostatic charge/discharge (ESC/ESD) protection, electromagnetic interference (EMI) shielding, resists, electroplating, corrosion protection of metals, and ultimately metal replacements, i.e. wiring, plastic microcircuits, conducting pastes for various interconnection technologies (solder alternative), etc. Many of the above applications especially those requiring high current capacity or those requiring good mechanical/physical properties have not yet been realized because the conductivity of the processible conducting polymers and the mechanical/physical properties of these polymers are not yet adequate for such applications.
The polyaniline class of conducting polymers are quite promising materials for many commercial applications. Great strides have been made in making these polymers processable. They are environmentally stable and allow chemical flexibility which in turn allows tailoring of their properties. A number of polyaniline coatings have been developed and commercialized for a number of applications such as ESD protection and corrosion protection.
In many of the current applications, polyaniline is generally applied as a coating to a specific substrate, e.g. metal, glass, plastic, etc.. For ESD protection or EMI Shielding, for example, the polyaniline is most commonly applied as a coating unto a plastic which has the physical and mechanical properties required for the particular application. Alternatively, the polyaniline can be incorporated as a conducting filler into a polymer matrix having properties appropriate for a given application. Thus, the polyaniline is used for its conducting properties and the substrate polymer or polymer matrix is used for its physical/mechanical properties. Polycarbonate is generally used to manufacture computer housings, keyboards, electronic component carriers, etc.. because it is a material that has excellent impact resistance, and overall mechanical/physical properties. Polyaniline or any of the other conducting polymers cannot be used alone to manufacture such parts because they do not have the appropriate physical and mechanical properties. They are relatively low molecular weight materials which tend to form brittle films having low impact resistance and relatively poor tensile properties.
K. T. Tzou and R. V. Gregory (Polymer Preprints, Vol. 1, 1994) has recently reported processing fibers from polyanilines. These fibers are quite promising for commercial applications. However, the tenacities and breaking elongations of these fibers do not yet compete with those attained with conventional plastics.
The conductivity of the polyanilines is generally on the low end of the metallic regime. The conductivity is on the order of 10.sup.0 S/cm. Some of the other soluble conducting polymers such as the polythiophenes, poly-para-phenylenevinylenes exhibit conductivity on the order of 10.sup.2 S/cm.
The conductivity (.sigma.) is dependent on the number of carriers (n) set by the doping level, the charge on the carriers (q) and on the interchain and intrachain mobility (.mu.) of the carriers. EQU .sigma.=n q .mu.
Generally, n (the number of carriers) in these systems is maximized and thus, the conductivity is dependent on the mobility of the carriers. To achieve higher conductivity, the mobility in these systems needs to be increased. The mobility, in turn, depends on the morphology of the polymer. The intrachain mobility depends on the degree of conjugation along the chain, presence of defects, and on the chain conformation. The interchain mobility depends on the interchain interactions, the interchain distance, the degree of crystallinity, etc. The mobility of the carriers between chains tends to limit the overall conductivity as the carriers need to hop from one chain to another which is an ineffective process. To enhance the conductivity, it would be necessary to provide a more effective interchain transport mechanism.
It is desirable to enhance the conductivity of the processable electrically conducting polymers and to enhance the physical and mechanical properties of both the conducting polymer precursors and the conducting polymers to allow them to more appropriately meet the needs of a number of applications.